The invention relates to a capacitor dielectric with inner blocking layers consisting of a polycrystalline ceramic body of a material with a perovskite structure on the basis of barium titanate with the general formula EQU (Ba.sub.1-x M.sub.x.sup.II)O.z(Ti.sub.1-y M.sub.y.sup.IV)O.sub.2
with M.sup.II =Ca, Sr, Ph and/or Mg and M.sup.IV =Zr, Sn and z assuming the values 1.005 to 1.05. The material contains at least two different doping substances one of which is preferably antimony, niobium, lanthanum or bismuth inside the crystallites and which causes predominantly n-type conduction; and the other, preferably copper, cobalt, nickel, iron or manganese in the surface layer of the crystallites and which causes predominantly p-type conduction. The proportion of the doping substance causing the n-type conduction is 1.5 to 2.5 times greater than the maximum doping quantity and the proportion of the substance causing the p-type conduction amounts to 0.01 to 0.15% by weight.
Such a capacitor dielectric is described in German published specification No. 1 614 605 and in the corresponding Great Britain patent specification No. 1 204 436 and U.S. Pat. No. 3,569,802.
One capacitor dielectric disclosed in these specifications has been commercially available for several years under the name SIBATIT 50.000 (SIBATIT, a registered trade mark) and has been technically proven many times. This capacitor dielectric can be used in the form of disks, tubes with circular and rectangular cross-section which are provided with metallic coatings (e.g. silver) as electrodes, and in the form of so-called "stack capacitors." Stack capacitors are capacitors in which thin layers of dielectric material are disposed one on another in alternate fashion with metallic layers extending to the edge alternately on different sides.
In order to insure maximum conductivity inside the grain in the simultaneous presence of the p-type doping substance despite the proportions of n-type doping substance being higher than those normally needed for maximum conductivity (maximum) doping), the above described specification proposes by way of preferred production processes that all substances be made to react together in oxide form. In such a case the conductivity inside the crystallite grains attains the highest possible values, while the p-type doping substance, particularly the copper (which can only be incorporated in the perovskite crystal lattice to a limited degree, if at all,) is essentially incorporated in the surface layer of the crystallites.
A ceramic material is described in British patent specification No. 1 047 057 as a capacitor dielectric which consists of a polycrystalline body of semi-conducting barium titanate and is made by applying substances such as iron, cobalt, manganese, copper among others on the surface of the body, whereupon the body thus prepared is heated. In the process, the named elements at the boundaries of the grains diffuse into the ceramic body along the grain boundaries therein. The semi-conducting properties of the barium titanate ceramic element are obtained through the body being made semiconducting either by reduction in a vacuum or in hydrogen gas or through bringing about so-called valency controlled semi-conduction by n-type doping substances. With the known capacitor dielectric and the method of production specified for it, the rate of diffusion plays a decisive part, and the diffusion process is relatively difficult to control. In addition it should be noted that with this method one first has to produce the semi-conducting body which then has to be brought into contact with the diffusion metals in further operational stages and subjected to heating to bring about the diffusion. The known method is unsuitable for the reproducibility required in mass production.
When one speaks of values for the dielectric constant (DK) in connection having a capacitor dielectric with inner blocking layers, one is always referring to apparent DK values, since establishing the DK from measurement of the capacitance of such a capacitor depends on the body as a whole having a high .epsilon., whereas in fact only the very thin p-n junctions at the grain boundaries become dielectrically effective. These grain boundaries exhibit a normal DK value for barium titanate but a DK which is increased many times results because of the reference to the casing as a whole.
In a capacitor dielectric the DK is not alone in playing a part with respect to capacitance level, for it is also necessary that the dependence of DK upon operating temperature, the tangent of the loss angle (loss factor), and the insulation and thus the charging capacity of the capacitor remain within certain limits.
This is largely already the case with the known capacitor dielectrics having inner blocking layers indicated above.
However efforts are being made both to improve the electrical properties and to obtain even more consistent reproducibility of these electrical values in mass production.
The time constant which is defined as the product of the capacitance of the capacitor and its insulation resistance expressed in M.OMEGA...mu. F or, after conversion, in seconds, is regarded as a measurement of the improvement in the electrical properties of electrical capacitors.
The time constant should always be indicated together with the value for the field strength at which measurement is made since both the capacitance and the insulation capacity depend upon the prevailing field strength.
Known blocking layer capacitors with inner blocking layers have time constants between 20 and 70 sec at a field strength of 100 V/mm and 2 to about 20 sec at a field strength of 200 V/mm.
In the present connection the improvement in the reproducibility of the electrical values in mass production means that the electrical values are obtained with reduced spreading when bodies having the same initial composition and undergoing the same preliminary treatment are sintered in different ovens, for instance.